1. Field of the Invention
The present invention relates to a driving force transmission apparatus used for an image reading apparatus such as an image scanner, a digital copying machine and the like for reading image information on a manuscript.
2. Description of the Related Art
In recent years, due to a high speed and a high resolution in reading an image by an image reading apparatus, in order to achieve both a high speed driving for reading the image at a high speed and a low speed driving for reading the image at a high resolution, a driving force transmission apparatus is required reading at a higher speed ratio. Further, the image reading apparatus has been made compact and inexpensive year by year, so that the driving force transmission apparatus therefor is required to be made compact and inexpensive as well as to be provided with a high speed ratio.
A conventional driving force transmission apparatus will be described below. FIG. 4 is a cross sectional view of an image reading apparatus using a conventional driving force transmission apparatus, FIG. 5 is a perspective view of the image reading apparatus using the conventional driving force transmission apparatus, and FIG. 6 is a perspective view of main construction members in the conventional driving force transmission apparatus.
In FIG. 4, reference numeral 1 denotes a frame body of a main body of an image scanner corresponding to an image reading apparatus, reference numeral 2 denotes a manuscript, reference numeral 3 denotes a manuscript table glass on which the manuscript is placed, reference numeral 4 denotes a manuscript pressing plate for adhering the manuscript 2 to the manuscript table glass 3, reference numeral 5 denotes a fluorescent lamp for lighting the manuscript 2, reference numeral 6 denotes a reflecting mirror, reference numeral 7 denotes a image forming lens for forming a light image, reference numeral 8 denotes a CCD in which some thousands of photo-electric conversion components for converting the light image formed by the image forming lens 7 into an electric signal in accordance with a photo-electric conversion are arranged on a line, and reference numeral 9 denotes an optical carriage corresponding to a moving body obtained by integrally constituting the reflecting mirror 6, the image forming lens 7 and the CCD 8.
In FIG. 5, reference numeral 10 denotes a rail for supporting the optical carriage 9 in such a manner as to move linearly, reference numeral 11 denotes a driving unit, reference numeral 12 denotes a driving pulley, reference numeral 13 denotes a transmission belt rotated by the driving pulley 12, and reference numeral 14 denotes a driven pulley tensioning the transmission belt 13 together with the drive pulley 12.
In FIG. 6, reference numeral 15 denotes a driving motor for generating a rotational force, reference numerals 16 and 17 denote a pulley, reference numeral 18 denotes a transmission belt tensioned between the pulleys 16 and 17, reference numeral 19 denotes a clutch, and reference numeral 20 denotes a shaft connected to the driving pulley 12. A rotation of the pulley 17 can be transmitted to the shaft 20 when the clutch 19 is energized. Reference numerals 21, 22, 23 and 24 denote a pulley, in which the pulley 21 is connected to the pulley 17 and the pulley 24 is connected to the shaft 20, respectively. Reference numerals 25 and 27 denote a transmission belt tensioned between the pulleys 21 and 22 and the pulleys 23 and 24, respectively, reference numeral 28 denotes a clutch, and reference numeral 29 denotes a shaft connected to the pulley 23. A rotation of the pulley 22 can be transmitted to the shaft 29 when the clutch 28 is energized.
With respect to the conventional driving force transmission apparatus structured in the manner mentioned above, an operation thereof will be described below with reference to FIGS. 4 to 6.
In FIGS. 4 and 5, a reflecting light after lighting the manuscript 2 by the fluorescent lamp 5 is reflected by the reflecting mirror 6 so as to form an image on the CCD 8 by the image forming lens 7, so that the image corresponds to a line of the manuscript 2 by the CCD 8. The driving unit 11 rotates the transmission belt 13, and the optical carriage 9 moves in a direction of an arrow A together with the transmission belt 13, whereby all the surface of the manuscript 2 can be read.
In this case, in order to read the manuscript 2, it is necessary to rotate the driving unit 11 at a high speed so as to move the optical carriage 9 at a high speed. Further, since a time for which the CCD 8 reads a line of the manuscript 2 is constant, in order to read the manuscript 2 at a high resolution, it is necessary to rotate the driving unit 11 at a low speed so as to move the optical carriage 9 at a low speed. By doing so, it is possible to read the manuscript 2 at a high density in a moving direction of the optical carriage 9.
Next, in FIG. 6, in the case that the clutch 19 is energized, the rotation of the driving motor 15 is transmitted to the pulley 16, the transmission belt 18, the pulley 17, the shaft 20 and the driving pulley 12, and the rotation of the driving motor 15 is reduced in accordance with a one-step speed reduction by the pulleys 16 and 17. Further, in the case that the clutch 28 is energized, the rotation of the driving motor 15 is transmitted to the pulley 16, the transmission belt 18, the pulley 17, the pulley 21, the transmission belt 25, the pulley 22, the shaft 29, the pulley 23, the transmission belt 26, the pulley 24, the shaft 20 and the driving pulley 12, and the rotation of the driving motor 15 is further reduced in accordance with a three-step speed reduction by the pulleys 16 and 17, the pulleys 21 and 22, and the pulleys 23 and 24.
As mentioned above, a speed ratio higher than a speed ratio of the driving motor 15 itself can be obtained by transmitting the rotation from the driving motor 15 in accordance with two systems of rotational force transmission paths.
However, in order to achieve a high speed ratio, in accordance with the conventional structure mentioned above, there is a problem that the driving unit 11 requires a large size and an expensive cost since a plurality of pulleys, transmission belts and clutches are required, and further, the image forming apparatus is made with an increased size and expense.